Cross-sectional fluorescent imaging system

ABSTRACT

A narrow x-ray beam is scanned across a cross section containing a relatively high atomic number material. When excited by the xray beam the material fluoresces and emits its characteristic radiation lines. The emission information along the entire line is collected to create a signal indicative of the line integral of the emission information. This line integral information, taken from many positions and angles in the cross section, is applied to a computer which reconstructs a cross-sectional image of the fluorescing material. The transmitted narrow beam, which represents the line integral of the density information, can simultaneously be used to create a reconstruction of the crosssectional density pattern.

nited States Patent [1 1 Macovski Dec. 16, 1975 CROSS-SECTIONALFLUORESCENT IMAGING SYSTEM Albert Macovski, 4100 Mackay Drive, PaloAlto, Calif. 94306 22 Filed: May 6, 1974 211 Appl. No.1 467,446

[76] Inventor:

OTHER PUBLICATIONS Fluorescent Thyroid Scanning Without Radioisotopes,Hoffer, et al., Radiology, Vol. 99, Apr., 1971. p. 117.

Primary Examiner craig E. Church 57 ABSTRACT A narrow x-ray beam isscanned across a cross section containing a relatively high atomicnumber material. When excited by the x-ray beam the material fluorescesand emits its characteristic radiation lines. The emission informationalong the entire line is collected to create a signal indicative of theline integral of the emission information. This line integralinformation. taken from many positions and angles in the cross section,is applied to a computer which reconstructs a cross-sectional image ofthe fluorescing material. The transmitted narrow beam, which representsthe line integral of the density information, can simultaneously be usedto create a reconstruction of the cross sectional density pattern.

13 Claims, 1 Drawing Figure I4 FLUORESCENT X-RAY BEAM "*7AITENUATION+ATTENUATION I 2| COMPENSATOR corvPENsAToR I RECON- 10 y k I STRUCTION-25 l I COMPUTER '4, I l I i i i w I I6 DISPLAY CROSS-SECTIONALFLUORESCENT IMAGING SYSTEM BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to systems for selectively imagingtrace amounts of specific materials. In a primary application theinvention relates to systems for imaging the fluorescent radiation frommaterials which are excited by x-rays.

2. Description of Prior Art Existing systems for imaging fluorescentradiation from excited materials usually include a collimated x-raysource for producing an x-ray beam. A selective detector is used havinga pulse-height analyzer which is tuned to the energy of the particularmaterial under study. This detector includes a collimator or focusingstructure for examining each region along the x-ray beam. The detectorand its collimator are normally scanned along the x-ray beam so as tosequentially measure the emission from each region along the beam. Thisis an inefficient process since, while emissions are occurring along theentire beam, only one portion at a time is being collected. In addition,the focusing collimators used to collect the radiation from a specificregion are relatively inefficient. A system of this type is described byP. B. Hoffer and A. Gottschalk in a paper entitled, Fluorescent ThyroidScanning Without Radioisotopes appearing in Radiology, Vol. 99, p. 117,April, 1971.

The imaging of fluorescent radiation is used in medicine for makingimages of the natural iodine in the thyroid or for imaging the selectiveuptake of administered materials in diseased areas such as tumors.Another important system for the diagnosis of diseased regions iscomputerized tomography which provides an accurate cross-sectionaldensity image. In this system accurate x-ray projections of a particularcross section are taken at many angles. This projection information,which is a number of line integrals of the density information, isapplied to a computer which reconstructs the desired density image. Asystem of this type is presently manufactured by EMI in England and isdescribed by J. Ambrose in the British Journal of Radiology, Vol. 46, p.1016, 1973. Although this instrument provides an accuratecross-sectional density image, many disease processes do not result in asignificant density change and are thus better diagnosed by detectingthe selective uptake of a material into the diseased region.

SUMMARY OF THE INVENTION An object of this invention is to provide anefficient system for imaging specific materials by their fluorescentradiation.

It is also an object of this invention to provide an efficient systemfor imaging fluorescent emissions by simultaneously collecting theintegrated emission information along the entire x-ray beam.

It is a further object of this invention to provide a combined displayindicating the density information and the amounts of a specificmaterial.

Briefly, in accordance with this invention, one or more unfocuseddetectors are used to collect the fluorescent radiation emitted alongthe entire x-ray beam. This information is collected for large numbersof positions and angles of the beam. This line-integralfluorescentinformation is applied to a computer to recon- 2 struct a crosssectional image of the fluorescing regions. The transmitted x-ray beamcan also be collected and used to provide a cross-sectional densityimage of the same area.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete disclosure of theinvention, reference may be made to the following detailed descriptionof an illustrative embodiment thereof which is given in conjunction withthe accompanying drawing, of which FIG. 1 is a schematic diagramillustrating an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT An understanding of the broadaspects of the invention may best be had by reference to FIG. 1 of thedrawings. A collimated x-ray source 10 can be derived using aconventional x-ray tube or an isotope source. These divergent sourcesare collimated using one or more metal absorbers having appropriateapertures to form a collimated beam 11. This beam is projected throughthe object under study 12 which would normally be the human anatomy. Theobject is known to contain amounts of a relatively high atomic numbermaterial which will fluoresce when excited by the x-ray beam 11 and emitits characteristic radiation. The material should have a relatively highatomic number so that the fluorescent emission will be sufficiently highenergy to escape the object without excessive attenuation. The materialsused are similar to those employed in nuclear medicine, exceptnon-radioactive, which are selectively taken up in diseased areas suchas tumors. Structures 13 in the object 12 represent regions which haveselectively taken up the material and will thus fluoresce at theircharacteristic radiation energy when excited.

In present fluorescent imaging systems, a focused collimator would beused to collect the radiation from a specific region along beam 11. Thisis an inefficient process since only a small portion of the totalfluorescent radiation is detected at any one time. This inefficiencyresults in excessive radiation to the patient and insufficient photonsto provide a good image. In the system described here, the radiation iscollected along the entire xray beam. No focusing structures arerequired. One or more gamma-ray detectors, such as 14 and 15, can beused to capture photons emitted along the length of the x-ray beam andcreate signal 21 representing the integrated emission along the beam.Ordinarily, an image could not be formed in this manner since theintegrated emission along the entire line is being measured rather thanthe emission from a small region. An image can be formed, however, byproviding these integrated measurements at a variety of angles. Thus thex-ray source 10 is linearly scanned, as shown by the dotted line, overan entire cross section of object 10. This same scan is repeated at aplurality of angles with the integrated fluorescent emission i=letectedat every position for every angle. Depending on the configuration ofgamma-ray detectors, l4 and 15; they can also'be moved to aid in thephoton collectiofi; Thus, for each region in a cross section of object10; 3 number of integrated emission measurements are made for lineswhich go through that region at different am The general mathematicalproblem of reconstructiflg a cross-sectional ii'ilage frem line-integralinformatl fi taken at many angles has been studied for many years inconnection with many applications. A comprehensive bibliography on thissubject is available from Richard Gordon of the National Institutes ofHealth, Building 31, Bethesda, Maryland. An example of some of theprocedures which can be used is given in a paper by G. T. Herman in, TwoDirect Methods for Reconstructing Pictures from Their Projections: AComparative Study, in Computer Graphics and Image Processing, Vol. I, p.l23l44, 1972. A general computer technique known as ART (AlgebraicReconstruction Technique) has received wide usage in this field. Themost recent use of reconstructions of this type is in a recentlyreleased brain-scanning instrument known as the EMI- Scanner. In thisinstrument the x-ray transmission through a cross section of the brainat a number of angles are recorded and applied to a computer. Thecomputer reconstructs the cross-sectional density image, with greataccuracy, using the ART technique. This system is described in a paperby J. Ambrose in the British Journal of Radiology, Vol. 46, 1973. Anyone of the many reconstruction algorithms can be applied to signal 21 tocreate a reconstructed cross-sectional image of fluorescent radiation.Leaving optional boxes 19 and 20 for subsequent discussion, the signal21 can be applied directly to reconstruction computer 16 where the crosssection is reconstructed from the integrated measurements by one of manyknown systems. The resultant fluorescent radiation information 23,indicating the presence of the materials 13, is displayed on display 17.

In the system described thusfar the x-ray beam transmitted through theobject is not measured or used in any way since only the resultantfluorescent radiation was detected. This transmitted beam can bedetected, however, and used to form a cross-sectional density image asdoes the EMI-scanner. This density information would be very valuable inmedical diagnosis for accurately defining the anatomy so that therelative positions of the diseased areas which take up the administeredmaterial can be well-defined. To create this cross-sectional densitydisplay, detector 18 is used to measure the transmitted beam at everyposition and angle of x-ray beam to form transmission signal 22. Thedetector 18 can either be mechanically scanned in synchronism with thesource 10 or an array of detectors can be used to collect thetransmitted beam at all of its positions. Transmission signal 22 isapplied to the same reconstruction computer 16 to reconstruct thedensity image signal 24 in one of the many known ways. The reconstructeddensity image signal 24 can be displayed in display 17 either separatelyor in conjunction with the fluorescent radiation signal 23. A number ofcombinational display configurations can be used including having thefluorescent radiation information displayed as a color overlay on theblack and white density information. In this way the diseased areas,which have selectively taken up the administered material, will becomevery apparent and be readily localized.

The detectors 14 and which measure the fluorescent radiation can be oneof a variety of gamma-ray detectors including scintillating crystals,proportional counters or solid state detectors. For improveddiscrimination they can employ energy spectrum analyzers for extractingthe energy region corresponding to the characteristic fluorescentemission energy of the material being studied. The energy spectrumanalyzer can be a pulse-height discriminator where the size of the de- 4tected pulse associated with each received photon indicates its energy.The detector 18 which measures the transmitted x-ray beam can also beany of the gammaray detectors previously listed.

The system has two potential sources of error; the attenuation of thefluorescent radiation through the object 12, and the attenuation of theX-ray beam 10 as it transverses the object. The fluorescent attenuationis determined by the energy of the radiation and the absorptioncoefficient of the various materials in the object. If a very largenumber of fluorescent radiation detectors, such as 13 and 14, are usedthe effect of this attenuation will be minimized since there willusually be some path or combination of paths of relatively lowattenuation. A compensation system 19 can be used to minimize the effectof the absorption. The object can be assumed to have a specificabsorption coefficient, for example that of soft tissue or water. Thespecific compensation would then be based on the geometry of thescanning and detecting configuration. For example, as shown in FIG. 1,the distance from the beam to the top edge of object 12 is y while thedistance to the bottom edge is d-y. Thus the transmission of theradiation to detector 14 will be e while that to detector 15 will be e F(d-y) where ,u. is the linear absorption coefficient of the assumedmaterial. Thus the effect of the attenuation can be minimized ifcompensation system 19 has a gain function [e e k. The size of theobject d can be initially provided to the system while the distance y ismade .available by the mechanical scanning system. The variablecompensating gain function can be accomplished by a small analog ordigital circuit arrangement.

A more exact compensation for fluorescent attenuation can be achievedthrough they separation of the various components of the fluorescentradiation. For example, excited materials produce K a and K emissions atspecific energies having specific relative amplitudes when emitted.Compensation system 19 can include a pulse-height discriminator' whichseparates and measures the K a and K B components. The measuredintensity at each energy region, I a and I B are given by l B Ge 2? Z lI where p. a an ,u. are the absorption-coefficients of the object at thetwo energies, Z is-the effective path length, I is the desired initialfluorescent output at the K 0 energy and C is known initial ratio of theK emission energy to that of the K a emission. Eliminating Z in the twoequations and solving for I we obtain which is an expression for thedesired emitted intensity independent of the path length. Thuscompensator 19 can be pre-programmed with the known values of ,u. a ,u.p and C to calculate the desired I signal from the measured values of Ia and I The second source of error is the attenuation that x-ray beam 11receives before exciting the fluorescent structures 13. One method ofcorrecting this error is to again assume that the object consists of agiven material such as soft tissue which is equivalent to water. Theattenuation of each beam after transversing a distance S is thus e I'- 5where ,u. is the assumed absorption coefficient of the object. Thus thecalculated fluorescent emission can be corrected at each region bycorrecting for the beam attenuationtothat region/Thus compensator willmodify the value of each region by dividing the calculated value bywhere n is the number of beams going through each region in the crosssection and S,, is the path length through the object to the particularregion. This summation is calculated for every region and used tocompensate the reconstructed fluorescent emission signal to obtaincorrected signal 23 which is applied to display 17.

A more accurate correction for the x-ray beam attenuation can beobtained by using the actual attenuation or density values in the crosssection rather than as sumed values. The actual density values have beencalculated in the reconstruction computer 16 and appear in signal 24.The attenuation of each beam to a given region is given by i by E u llwhere g,,(S represents the attenuation of each of the n beams whichintersect point S The value of is calculated for every region i in thecross section and becomes correction signal 25. Compensator 20 divideseach calculated fluorescent emission signal by the correction signal toobtain the corrected fluorescent emission signal 23 which is applied todisplay 17.

While particular embodiments of the invention have been shown anddescribed, it will of course be understood that the invention is notlimited thereto since many modifications in the x-ray scanningarrangements and electronic processing can be made. It is contemplatedthat the appended claims will cover any such modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for imaging fluorescent radiation from a specific materialin a cross section of an object comprising:

an x-ray beam directed through the cross section of the object;

means for translating the x-ray beam so that all regions of the crosssection are excited at a plurality of angles:

means for detecting the integrated fluorescent radiation emitted alongthe entire x-ray beam at each position of the x-ray beam and forming aplurality of integrated emission signals:

a computer for reconstructing the fluorescent radiation information ofthe cross section of the object from the plurality of integratedemission signals: and

means for displaying the reconstructed fluorescent radiationinformation.

2. Apparatus as recited in claim 1 including means for correcting theplurality of integrated emission signals for the attenuation of thefluorescent radiation in the object before reaching the detector means.

3. Apparatus as recited in claim 2 wherein the means for correcting forattenuation includes an energy spectrum analyzer for separatelymeasuring the K a and K components of the fluorescent radiation andmeans for comparing their amplitudes to their initial emitted relativeamplitudes to compute the required correction.

4. Apparatus as recited in claim 2 wherein the means for correcting forattenuation includes a pre-programmed compensator based on an objecthaving an assumed size and absorption where the pre-programmedcompensator is varied according to the distance of the x-ray beam to theedge of the object.

5. Apparatus as recited in claim 1 wherein the means for detecting thefluorescent radiation includes a plurality of gamma-ray detectorswhereby a large percentage of the radiation is received.

6. Apparatusas recited in claim 1 wherein the means for detecting thefluorescent radiation includes an energy spectrum analyzer for isolatingthe energy level of the fluorescent radiation from the specificmaterial.

7. Apparatus as recited in claim 6 wherein the spectrum analyzer is apulse-height discriminator.

8. Apparatus as recited in claim 1 including: means for detecting thexray beam after transmission through the object at each position of thex-ray beam to form a plurality of transmission signals;

means for computing the density information of the cross section of theobject from the plurality of transmission signals; and

means for displaying the reconstructed density information.

9. Apparatus as recited in claim 8 wherein the means for displaying thereconstructed density information includes a composite display of thedensity and fluorescent radiation information.

10. Apparatus as recited in claim 9 wherein the composite display is acolor display having the density and fluorescent radiation informationin different colors.

11. Apparatus as recited in claim 1 including means for correcting thefluorescent radiation information for the attenuation of the x-ray beamin the object.

12. Apparatus as recited in claim 11 wherein the means for correctingfor the attenuation of the x-ray beam includes a pre-programmedcompensator based on an object having an assumed absorption where theattenuation at each position of the x-ray beam to each region of thecross section is calculated.

13. Apparatus as recited in claim 11 wherein the means for correctingfor the attenuation of the x-ray beam comprises:

means for detecting the x-ray beam after transmission through the objectto form a plurality of transmission signals;

7 8 mation at each region of the cross section usmg the means forcomputing the attenuation of the x-ray Computed attenuation of the X raybeam to that beam to each region of the cross section from the pluralityof transmission signals; and

means for correcting the fluorescent radiation infor-

1. Apparatus for imaging fluorescent radiation from a specific materialin a cross section of an object comprising: an x-ray beam directedthrough the cross section of the object; means for translating the x-raybeam so that all regions of the cross section are excited at a pluralityof angles; means for detecting the integrated fluorescent radiationemitted along the entire x-ray beam at each position of the x-ray beamand forming a plurality of integrated emission signals; a computer forreconstructing the fluorescent radiation information of the crosssection of the object from the plurality of integrated emission signals;and means for displaying the reconstructed fluorescent radiationinformation.
 2. Apparatus as recited in claim 1 including means forcorrecting the plurality of integrated emission signals for theattenuation of the fluorescent radiation in the object before reachingthe detector means.
 3. Apparatus as recited in claim 2 wherein the meansfor correcting for attenuation includes an energy spectrum analyzer forseparately measuring the K and K components of the fluorescent radiationand means for comparing their amplitudes to their initial emittedrelative amplitudes to compute the required correction.
 4. Apparatus asrecited in claim 2 wherein the means for correcting for attenuationincludes a pre-programmed compensator based on an object having anassumed size and absorption where the pre-programmed compensator isvaried according to the distance of the x-ray beam to the edge of theobject.
 5. Apparatus as recited in claim 1 wherein the means fordetecting the fluorescent radiation includes a plurality of gamma-raydetectors whereby a large percentage of the radiation is received. 6.Apparatus as recited in claim 1 wherein the means for detecting thefluorescent radiation includes an energy spectrum analyzer for isolatingthe energy level of the fluorescent radiation from the specificmaterial.
 7. Apparatus as recited in claim 6 wherein the spectrumanalyzer is a pulse-height discriminator.
 8. Apparatus as recited inclaim 1 including: means for detecting the x-ray beam after transmissionthrough the object at each position of the x-ray beam to form aplurality of transmission signals; means for computing the densityinformation of the cross section of the object from the plurality oftransmission signals; and means for displaying the reconstructed densityinformation.
 9. Apparatus as recited in claim 8 whereiN the means fordisplaying the reconstructed density information includes a compositedisplay of the density and fluorescent radiation information. 10.Apparatus as recited in claim 9 wherein the composite display is a colordisplay having the density and fluorescent radiation information indifferent colors.
 11. Apparatus as recited in claim 1 including meansfor correcting the fluorescent radiation information for the attenuationof the x-ray beam in the object.
 12. Apparatus as recited in claim 11wherein the means for correcting for the attenuation of the x-ray beamincludes a pre-programmed compensator based on an object having anassumed absorption where the attenuation at each position of the x-raybeam to each region of the cross section is calculated.
 13. Apparatus asrecited in claim 11 wherein the means for correcting for the attenuationof the x-ray beam comprises: means for detecting the x-ray beam aftertransmission through the object to form a plurality of transmissionsignals; means for computing the attenuation of the x-ray beam to eachregion of the cross section from the plurality of transmission signals;and means for correcting the fluorescent radiation information at eachregion of the cross section using the computed attenuation of the x-raybeam to that region.